FIG. 7 is a perspective view showing an internal structure of package 1300 incorporating a conventional MEMS device. FIG. 7 shows a state in which a lid of package 1300 is opened. Package 1300 is mounted on substrate 1500. Sensor chip 1100 and integrated circuit 1200 such as an ASIC (Application Specific Integrated Circuit) are mounted on package 1300. The MEMS device is accommodated in sensor chip 1100. Integrated circuit 1200 carries out various operations based on an output from sensor chip 1100. Terminals 1400 are extracted from package 1300 and connected to substrate 1500.
Hereinafter, a capacitance type acceleration sensor as a MEMS device is described. FIG. 8A is a vertical sectional view of conventional acceleration sensor 210. FIG. 8B is a horizontal sectional schematic view of conventional acceleration sensor 210. FIG. 8C is a top view of movable body 101 of conventional acceleration sensor 210. FIG. 8D is a bottom view of movable body 101 of conventional acceleration sensor 210.
Capacitance type acceleration sensor 210 includes movable body 101 and electrode substrates 122a and 122b. 
Movable body 101 includes movable section 111, frame 133, and beams 16a and 16b. Frame 133 is an outer frame surrounding movable section 111. Movable section 111 is supported by frame 133 around beams 16a and 16b as a rotary shaft.
As shown in FIG. 8C, movable section 111 includes shaft 145 that is an extension portion of beams 16a and 16b, and swing sections 144a and 144b that are places other than shaft 145 of movable section 111. Swing section 144a is a cube, and swing section 144b is a cube having opening 111d at an opposite side to a surface facing fixed electrode 112b. 
Frame 133 is formed of silicon (Si). Movable section 111 is formed of SOI (Silicon on Insulator). Specifically, movable section 111 is formed by sandwiching oxide film 111b such as SiO2 between Si layer 111a and Si layer 111c. 
Electrode substrates 122a and 122b are disposed to both surfaces of movable body 101. A periphery of movable body 101 is bonded to peripheries of electrode substrates 122a and 122b by anodic bonding. Electrode substrate 122a includes substrate section 120a, lead electrodes 114a and 114b, and fixed electrodes 112a and 112b. Substrate section 120a is formed of glass. Lead electrodes 114a and 114b are formed of Si. Electrode substrate 122b includes substrate section 120b formed of glass. Fixed electrodes 112a and 112b are a metal thin film formed by, for example, sputtering. Fixed electrode 112a is formed on electrode substrate 122a in at least a part of a region facing swing section 144a. Fixed electrode 112b is formed on electrode substrate 122a in at least a part of a region facing swing section 144b. Lead electrodes 114a and 114b are embedded in substrate section 120a, and thereby potential of fixed electrodes 112a and 112b can be led to the upper surface of electrode substrate 122a. 
When movable section 111 swings by acceleration, capacitance between fixed electrode 112a and movable section 111 as well as capacitance between fixed electrode 112b and movable section 111 are changed. For example, capacitance C can be calculated from C=∈S/d where ∈ is a dielectric constant of a substance, S is an area of electrodes sandwiching the substance, and d is a gap between the electrodes. Since capacitance C is changed when movable section 111 swings by acceleration, by calculating the differential capacity by integrated circuit 1200, the acceleration can be detected.
That is to say, acceleration sensor 210 detects displacement of movable section 111 from the change in the capacitance between movable section 111 and fixed electrodes 112a and 112b, and then detects acceleration based on the detected displacement.
A plurality of projecting stoppers 134 is formed on movable section 111 at a surface facing fixed electrodes 112a and 112b. Formation of stoppers 134 can suppress damage due to collision of movable section 111 with fixed electrodes 112a and 112b even when large acceleration is applied to movable section 111.
Bonding portions between electrode substrates 122a and 122b and frame 133 are substrate sections 120a and 120b, respectively and are formed of glass. Bonding portions between movable body 101 and electrode substrates 122a and 122b are formed of silicon (Si). Electrode substrates 122a and 122b and movable body 101 are bonded to each other by using anodic bonding. However, during the anodic bonding, depending on an applied voltage, an electrostatic attraction force is generated between glass and Si. With this electrostatic attraction force, a part of movable section 111 may be attracted to electrode substrate 122a side, and bonded thereto.
FIG. 9A is a vertical sectional view of another conventional acceleration sensor 212. FIG. 9B is a horizontal sectional schematic view of the another conventional acceleration sensor 212. Recess 130 is provided to movable section 111 at a region facing a portion interposed between fixed electrode 112a and fixed electrode 112b (hereinafter, referred to as “facing region”). Recess 130 is formed by thinning at least a part of the facing region of movable section 111. By providing recess 130, an electrostatic attraction force can be reduced. When the electrostatic attraction force is made to be smaller than an elastic force, it is possible to suppress bonding of movable section 111 to substrate section 120a. 
However, in a small chip, beams 16a and 16b are required to be made extremely thin. As a result, beams 16a and 16b may be deformed due to the electrostatic attraction force generated during anodic bonding, and beams 16a and 16b may be bonded to glass.
In the case where beams 16a and 16b are made extremely thin, when movable body 101 and electrode substrate 122a are bonded to each other by anodic bonding at a voltage about 400V, beams 16a and 16b are attracted to substrate section 120a by the electrostatic attraction force generated at the time of anodic bonding. Accordingly, beams 16a and 16b may be deformed, and beams 16a and 16b may be bonded to substrate section 120a. 
That is to say, when movable body 101 and electrode substrate 122a are bonded to each other by anodic bonding, an electrostatic attraction force is generated between glass and Si by the applied voltage. With this electrostatic attraction force, a part of movable section 111 formed of Si may be attracted to glass, and thus bonded to glass. In particular, in a small chip, beams 16a and 16b are being thinned. As a result, beams 16a and 16b may be deformed due to the electrostatic attraction force generated during anodic bonding, and beams 16a and 16b may be bonded to glass. Prior art literature mentioned above includes, for example, PTL 1.